Nuclear receptors are members of a superfamily of transcription factors. The members of this family share structural similarities and regulate a diverse set of biological effects (Olefsky, J. M. J. Biol. Chem. 2001, 276(40), 36863-36864). Ligands activate or repress these transcription factors that control genes involved in metabolism, differentiation and reproduction (Laudet, V. and H. Gronmeyer. The Nuclear Receptor Factbooks. 2002, San Diego: Academic Press). Presently, the human genome project has identified about 48 members for this family and cognate ligands have been identified for about 28 of them (Giguere, V. Endocrine Rev. 1999, 20(5), 689-725). This protein family is composed of modular structural domains that can be interchanged within the members of the family without loss of function. A typical nuclear receptor contains a hypervariable N-terminus, a conserved DNA binding domain (DBD), a hinge region, and a conserved ligand-binding domain (LBD). The function of the DBD is targeting of the receptor to specific DNA sequences (Nuclear Hormone Receptor (NHR) response elements or NREs), and the function of the LBD is recognition of its cognate ligand. Within the sequence of the nuclear receptor there are regions involved in transcriptional activation. The Activation Function 1 (AF-1) domain is situated at the N-terminus and constitutively activates transcription (Rochette-Egly, C. et al. Cell 1997, 90, 97-107; Rochette-Egly, C. et al. Mol. Endocrinol. 1992, 6, 2197-2209), while the Activation Function 2 (AF-2) domain is embedded within the LBD and its transcriptional activation is ligand dependent (Wurtz, J. M. et al. Nat. Struct. Biol. 1996, 3, 87-94). Nuclear receptors can exist as monomers, homodimers or heterodimers and bind to direct or inverted nucleotide repeats (Laudet and Gronmeyer, 2002; Aranda, A. and A. Pascual. Physiol. Rev. 2001, 81(3), 1269-1304).
The members of this family exist either in an activated or repressed basal biological state. The basic mechanism of gene activation involves ligand dependent exchange of co-regulatory proteins. These co-regulatory proteins are referred to as co-activators or co-repressors (McKenna, L. J. et al. Endocrine Rev. 1999, 20, 321-344). A nuclear receptor in the repressed state is bound to its DNA response element and is associated with co-repressor proteins that recruit histone de-acetylases (HDACs) (Jones, P. L. and Y. B. Shi. Curr. Top. Microbiol. Immunol. 2003, 274, 237-268). In the presence of an agonist there is an exchange of co-repressors with co-activators that in turn recruit transcription factors that assemble into an ATP dependent chromatin-remodeling complex. Histones are hyper-acetylated, causing the nucleosome to unfold, and repression is alleviated. The AF-2 domain acts as the ligand dependent molecular switch for the exchange of co-regulatory proteins. In the presence of an agonist the AF-2 domain undergoes a conformational transition and presents a surface on the LBD for interaction with co-activator proteins. In the absence of an agonist or in the presence of an antagonist the AF-2 domain presents a surface that promotes interactions with co-repressor proteins. The interaction surfaces on the LBD for both co-activators, and co-repressors overlap and provide a conserved molecular mechanism for gene activation or repression that is shared by the members of this family of transcription factors (Xu, H. E. et al. Nature 2002, 415 (6873), 813-817).
Natural ligands that modulate the biological activity of nuclear receptors have been identified for only approximately one half of known nuclear receptors. Receptors for which no natural ligand has been identified are termed “orphan receptors.” The discovery of ligands or compounds that interact with an orphan receptor will accelerate the understanding of the role of the nuclear receptors in physiology and disease and facilitate the pursuit of new therapeutic approachesEstrogen related receptors (ERRs) constitutes a sub-class of these receptors where no ligand has been identified.
ERR-α (also known as ERR-1), an orphan receptor, is the first of the three identified members of the estrogen receptor related subfamily of orphan nuclear receptors (ERR-α, β, γ). The ERR subfamily is closely related to the estrogen receptors (ER-α and ER-β). ERR-α and ERR-β were first isolated by a low stringency hybridization screen (Giguere, V. et al. Nature 1988, 331, 91-94) followed later with the discovery of ERR-γ (Hong, H. et al. J. Biol. Chem. 1999, 274, 22618-22626). The ERRs and ERs share sequence similarity with the highest homology observed in their DBDs, approximately 60%, and all interact with the classical DNA estrogen response element. Recent biochemical evidence suggested that the ERRs and ERs share target genes, including pS2, lactoferin, aromatase and osteopontin, and share co-regulator proteins (Giguere, V. Trends in Endocrinol. Metab. 2002, 13, 220-225; Vanacker, J. M. et al. EMBO J. 1999, 18, 4270-4279; Kraus, R. J. et al. J. Biol. Chem. 2002, 272, 24286-24834; Hong et al., 1999; Zhang, Z. and C. T. Teng. J. Biol. Chem. 2000, 275, 20387-20846). Therefore, one of the main functions of ERR is to regulate the response of estrogen responsive genes. The effect of the steroid hormone estrogen is primarily mediated in the breast, bone and endometrium. Thus, the identification of compounds that will interact with ERRs should provide a benefit for the treatment of bone related disease, breast cancer and reproduction.
ERR-α is shown to be present both in normal and breast cancer tissue (Ariazi, E. A. et al. Cancer Res. 2002, 62, 6510-6518). It has been reported that the main function of ERR-α in normal breast tissue is that of a repressor for estrogen responsive genes. In breast cancers or cell lines that are non-estrogen responsive (ER-α negative), ERR-α has been reported to be in an activated state (Ariazi et al., 2002). Therefore, compounds that will interact with ERR-α may be useful agents for the treatment of breast cancer that is ER-α negative and non-responsive to classical anti-estrogenic therapy, or may be used as an adjunct agent for anti-estrogen responsive breast cancers. These agents may act as antagonists by reducing the biological activity of ERR-α in these particular tissues.
Many post-menopausal women experience osteoporosis, a condition that is a result of the reduction of estrogen production. Reduction of estrogen levels results in an increase of bone loss (Turner, R. T. et al. Endocrine Rev. 1994, 15(3), 275-300). An anabolic effect on bone development has been observed on the administration of estrogens to postmenopausal patients with osteoporosis (Pacifici, R. J. Bone Miner. Res. 1996, 11(8), 1043-1051) but the molecular mechanism is unknown since ER-a and ER-b knock-out animals have minor skeletal defects, where the action of estrogens is typically mediated (Korach, K. S. Science 1994, 266, 1524-1527; Windahl, S. H. et al. J. Clin. Invest. 1999, 104(7), 895-901). Expression of ERR-α in bone is regulated by estrogen (Bonnelye, E. et al. Mol. Endocrin. 1997, 11, 905-916; Bonnelye, E. et al. J. Cell Biol. 2001, 153, 971-984). ERR-α is maintained throughout osteoblast differentiation stages. Over-expression of ERR-α in rat calvaria osteoblasts, an accepted model of bone differentiation, results in an increase of bone nodule formation, while treatment of rat calvaria osteoblasts with ERR-α antisense results in a decrease of bone nodule formation. ERR-α also regulates osteopontin, a protein believed to be involved in bone matrix formation. Therefore compounds that will modulate ERR-α by increasing its activity can have an anabolic effect for the regeneration of bone density and provide a benefit over current approaches that prevent bone loss, but have no anabolic effect. Such compounds can enhance the activity of the receptor by two possible mechanisms: i) enhancing the association of the receptor with proteins that enhance its activity or improve the stability of the receptor; and ii) increasing the intracellular concentrations of the receptor and consequently increasing its activity. Conversely, with respect to bone diseases that are a result of abnormal bone growth, compounds that will interact with ERR-α and decrease its biological activity may provide a benefit for the treatment of these diseases by retarding bone growth. Antagonism of the association of the receptor with co-activator proteins decreases the activity of the receptor.
ERR-α is also present in cardiac, adipose, and muscle tissue and forms a transcriptional active complex with the PGC-1 co-activator family, co-activators implicated with energy homeostasis, mitochondria biogenesis, hepatic gluconeogenesis and in the regulation of genes involved in fatty acid beta-oxidation (Kamei, Y. et al. Proc. Natl. Acad. Sci. USA 2003, 100(21), 12378-12383). ERR-α regulates the expression of the medium chain acyl-CoA dehydrogenase promoter (MCAD). Medium chain acyl-CoA dehydrogenase is a gene involved in the initial reaction in fatty acid beta-oxidation. It is believed that in the adipose tissue ERR-α regulates energy expenditure through the regulation of MCAD (Sladek, R. et al. Mol. Cell. Biol. 1997, 17, 5400-5409; Vega, R. B. and D. P. Kelly. J. Biol. Chem. 1997, 272, 31693-31699). In antisense experiments in rat calvaria osteoblasts, in addition to the inhibition of bone nodule formation, there was an increase in adipocyte differentiation markers including aP2 and PPAR-γ (Bonnelye, E. et al. Endocrinology 2002, 143, 3658-3670). Recently an ERR-α knockout model has been described that exhibited reduced fat mass relative to the wild type and DNA chip analysis data indicated alteration of the expression levels of genes involved in adipogenesis and energy metabolism (Luo, J. et al. Mol. Cell. Biol. 2003, 23(22), 7947-7956). More recently it has been shown that ERR-α regulates the expression of endothelial nitric oxide synthase, a gene that has a protective mechanism against arteriosclerosis (Sumi, D. and L. J. Ignarro. Proc Natl. Acad. Sci. 2003, 100, 14451-14456). The biochemical evidence supports the involvement of ERR-α in metabolic homeostasis and differentiation of cells into adipocytes. Therefore, compounds interacting with ERR-α can affect energy homeostasis and may therefore provide a benefit for the treatment of obesity and metabolic syndrome related disease indications, including arteriosclerosis and diabetes (Grundy, S. M. et al. Circulation 2004, 109(3), 433-438).
There is a continuing need for new ERR-α modulators. There is also a need for ERR-α modulators useful for the treatment of conditions including but not limited to ankylosing spondylitis, artherosclerosis, arthritis (such as rheumatoid arthritis, infectious arthritis, childhood arthritis, psoriatic arthritis, reactive arthritis), bone-related diseases (including those related to bone formation), breast cancer (including those unresponsive to anti-estrogen therapy), cardiovascular disorders, cartilage-related disease (such as cartilage injury/loss, cartilage degeneration, and those related to cartilage formation), chondrodysplasia, chondrosarcoma, chronic back injury, chronic bronchitis, chronic inflammatory airway disease, chronic obstructive pulmonary disease, diabetes, disorders of energy homeostasis, gout, pseudogout, lipid disorders, metabolic syndrome, multiple myeloma, obesity, osteoarthritis, osteogenesis imperfecta, osteolytic bone metastasis, osteomalacia, osteoporosis, Paget's disease, periodontal disease, polymyalgia rheumatica, Reiter's syndrome, repetitive stress injury, hyperglycemia, elevated blood glucose level, and insulin resistance.